McCollough Effect

Celeste McCollough

The McCollough effect is a phenomenon of human visual perception discovered by American psychologist Celeste McCollough in 1965 in which colorless gratings appear colored contingent on the orientation of the gratings. It is an aftereffect requiring a period of induction to produce it. For example, if someone alternately looks at a red horizontal grating and a green vertical grating for a few minutes, a black-and-white horizontal grating will then look greenish and a black-and-white vertical grating will then look pinkish.

The effect is remarkable because it is very long-lasting. McCollough originally reported that aftereffects may last for an hour or more, but they can persist much longer. A 1975 study found that 15 minutes of induction can lead to an effect lasting three and a half months.

The effect is different from colored afterimages, which appear superimposed on whatever is seen and which are quite brief. The McCollough effect depends on retinal orientation (tilting the head to the side by 45 degrees can induce the effect). However, there is also some evidence of binocular interactions (inducing the effect with one eye leads to no effect being seen with the other eye).

Any aftereffect requires a period of induction (or adaptation) with an induction stimulus (or, in the case of the McCollough effect, induction stimuli). It then requires a test stimulus on which the aftereffect can be seen. The McCollough induction stimuli can have any different colors. The effect is strongest, however, when the colors are complementary, such as red and green, or blue and orange.

The effect is also optimal when the thickness of the bars in the induction stimulus matches that of those in the test stimulus (i.e., the effect is tuned, albeit broadly, to spatial frequency). This property led to non-redundant effects being reported by people who had used computer monitors with uniformly colored phosphors to do word processing. These monitors were popular in the 1980s, and commonly showed text as green on black. People noticed later when reading text of the same spatial frequency, in a book say, that it looked pink. Also a horizontal grating of the same spatial frequency as the horizontal lines of the induction text (such as the horizontal stripes on the letters ‘IBM’ on the envelope for early floppy disks) looked pink.

A variety of similar aftereffects have been discovered not only between pattern and color contingencies, but between movement/color, spatial frequency/color and other relationships. All such effects may be referred to as McCollough Effects or MEs. A functional explanation of MEs has been posited in the form of an error-correcting device (ECD) whose purpose is to maintain an accurate internal representation of the external world. Consistent pairings of color and oriented lines are not found frequently in natural environments, thus consistent pairing may indicate pathology of the eye. An ECD might compensate for such pathology by adjusting the appropriate neurons to a neutral point in adaptation to orientation contingent color.

Another explanation points to the contribution of classical conditioning to normal homeostatic regulation. MEs are explained by the same mechanisms as pharmacological withdrawal symptoms, thus the ‘pharmacological CR is expressed as pharmacological adaptation (tolerance) in the presence of the drug, and withdrawal symptoms in the absence of the drug’ and the ‘chromatic CR is expressed as chromatic adaptation in the presence of color, and the ME in the absence of color.’ By this account MEs are of no adaptive value, but have been selected for as a domain-general ability to anticipate events.

Neurophysiological explanations of the effect have variously pointed to the adaptation of cells in the visual pathway designed to correct for chromatic aberration of the eye, to adaptation of cells in the visual cortex jointly responsive to color and orientation (this was McCollough’s explanation), to processing within higher centers of the brain (including the frontal lobes), and to learning and memory. In 2006, the explanation of the effect was still the subject of debate, although there was a consensus in favor of McCollough’s original explanation.

In 2008, a similar effect with different results was discovered, and has been termed the ‘anti-McCollough effect.’ It is induced by alternating pairings of gratings in parallel alignment, one achromatic (black and white) and the other black and a single color (say black and red). If the color used was red, then after the induction phase the achromatic grating appeared slightly red.

This effect is distinct from the classical effect in three important regards: the perceived color of the aftereffect is the same as the inducer’s color, the perceived color of the aftereffect is weaker than the classical effect, and the aftereffect shows complete interocular transfer. Like the classic effect, the anti-McCollough effect (AME) is long lasting. Despite producing a less saturated illusory color, the induction of an AME may override a previously induced ME.

Given that AMEs do transfer interocularly, it is reasonable to suppose that they must occur in higher, binocular regions of the brain. Despite producing a less saturated illusory color, the induction of an AME may override a previously induced ME, providing additional weight to the argument that AMEs occur in the higher visual areas than MEs.

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